I have a Dell XPS 13 9343. Make sure you are changing the correct option. There are two different things that say KbdBacklight. I had to scroll to see the second option. I accidentally changed the first option when I did it the first time.

This sounds somewhat familiar to a problem that I have faced earlier this year when I purchased a Cooler Master keyboard + mouse Devastator combo.

The gimmic was that both the mouse and the keyboard had flashing lights dimming from/to all colors in the rainbow.

So I plugged the mouse in and it worked fine. But the keyboard had 0 light poping up on it yet it worked fine. After some mixed interactions with (I even claimed a RMA thinking it was defective), my girlfriend simply plugged it into a windoze 10 machine and THE LIGHTS WENT ON! Plugging it back however into a Linux host brought the same initial result: keyboard worked but not the lights.

At ths point I simply lost all hope of getting it working and reverted back to a “regular” keyboard.

This is probably not the answer you were looking for but, hey, true story nonetheless.

Let’s bring this VERY USEFUL Thread alive.
I havent any Dell. Mine’s Asus Zenbook UX32VD.
That being said, the fix you proposed works for me, independently from what line I change from “allow” to “deny”. (Works with first as well as second line)
Problem: The Zenbook has no ON and OFF backlight switch. You can turn it off and on using Fn + LIght up or light down, until it is completely off, or completely on (Has 4 level OFF > LOW ON > MIDDLE ON > HIGH ON)
As i said, it works, but completely disable the possibility to turn the keyboard light on when it’s needed.
In the Zenbook, even when the screen dim down to save power, and then you dim up again moving the mouse, the keyboard light turn on again, even from OFF state.

Note that Manjaro Gnome gets this right: fn + f3 and fn +f4 work as they should, and it always remembers the brightness level how you left it, and no fix/hack in /etc/dbus-1/system.d/org.freedesktop.UPower.conf was ever needed.

I am indebted to you not so CrazyDesi. On my laptop (Dell Inspiron convertible 15-5578) the keyboard backlight turning on after turning off was really getting to me and one thing that was p***ing me off with Linux Mint. Normally in the typing of this message; backlights would have turned on a couple of times; didn’t happen so thank you very much.

The org.freedesktop.UPower.conf workaround worked on my Dell XPS 9380 (with Mint 19.1 MATE, which is based on Ubuntu 18.04).

The issue I was suffering was that on logon, the keyboard backlight would turn on, even if it's disabled in the BIOS.

Somebody mentioned a fix in upower; as experiment, I've upgraded the upower packages (libupower-glib3 and upower) in my system to the Ubuntu 19.04 versions, however, they didn't solve the problem, so I expect this issue not to be fixed for a long time.